1. Field of the Invention
The present invention pertains to mechanically assisted ventilatory support and more particularly to a ventilation system which synchronizes the delivery of positive pressure ventilation and negative pressure ventilation to provide improved total ventilatory support to a patient.
2. Description of the Related Art
Mechanically assisted ventilatory support may be delivered to a patient either invasively or non-invasively. Invasive ventilation generally requires endotracheal intubation (i.e., insertion of a breathing tube into the patient's airway) or a tracheostomy (i.e., creation of an artificial opening in the patient's trachea to which a breathing tube is inserted). Non-invasive ventilation (NIV) refers to the delivery of mechanically assisted ventilatory support to a patient without endotracheal intubation or tracheostomy.
NIV was originally delivered to the patient using negative pressure systems having a negative pressure generator and a thoracic interface such as, without limitation, a body tank (also referred to as an “iron lung”), a chest cuirass (also referred to as a “tortoise shell”), and a body wrap (also referred to as a “jacket”). Negative pressure generators are generally designed to provide a negatively pressured source of gas to the thoracic interface. The thoracic interface converts the negatively pressured source of gas into a negative extrathoracic pressure which is communicated to the patient.
Negative pressure generators are generally controlled such that the negative extrathoracic pressure is intermittently applied to the patient. Application of negative extrathoracic pressure causes the patient's chest cavity to expand thereby creating a sub-atmospheric pressure within the patient's lungs. A breathing gas (e.g., air), which is generally at atmospheric pressure, is drawn into the patient's airway and inflates the lungs. Removal of the negative extrathoracic pressure allows the patient's chest cavity to naturally recoil thereby expelling the breathing gas from the lungs.
Negative pressure systems have several limitations. For example, the thoracic interface requires seals around the patient's neck and/or thorax. These seals are difficult to maintain. As a result, it is difficult to efficiently communicate the negative extrathoracic pressure to the patient. Additionally, negative pressure systems are often limited with respect to triggering and cycling of breaths due to the large compliance of the thoracic interface.
More recently, positive pressure systems have replaced negative pressure systems as the preferred manner of providing NIV. Positive pressure systems may have a positive pressure generator and a patient interface. Positive pressure generators include, without limitation, ventilators, pressure support devices (such as a continuous positive airway pressure device, or CPAP device), variable pressure devices (e.g., BiPAP®, Bi-Flex®, or C-Flex™ devices manufactured and distributed by Respironics, Inc. of Pittsburgh, Pa.), and auto-titration pressure support systems. Positive pressure devices are generally designed to provide a positively pressured source of breathing gas to the patient's airway via the patient interface (e.g., a nasal mask, a full-face mask, a total face mask, or a mouthpiece). The positively pressured breathing gas prevents the patient's airway from collapsing (i.e., splints open the patient's airway) so that respiration remains uninterrupted.
Positive pressure systems were developed to overcome many of the problems associated with, negative pressure systems. Positive pressure systems, however, have their own distinct limitations. For example, the effectiveness of a positive pressure system is limited by the ability to maintain a low level of leakage between the patient interface and the patient's face. Additionally, the pressure at which the supply of breathing gas is delivered is limited by a patient's relatively low glottic opening pressure, which if exceeded, may result in gastric distention. Furthermore, positive pressure ventilatory support is lost when the patient removes the patient interface, for example, to talk, eat, or drink.
Accordingly, a need exists for an improved ventilation system which overcomes these and other problems associated with known systems and which provides improved ventilatory support.